In a process of producing semiconductor devices, a number of semiconductor chips of the same inner pattern are formed on a wafer. The chip pattern, i.e. the inner pattern of the chip, is formed by arranging many pattern components in prescribed positions. Each of the pattern components has, for example, a rectangular shape. Several photomasks are successively used in the process of forming chips on a wafer. The mask pattern layout of the photomask precisely coincides with the chip pattern layout. The wafer is exposed through this photomask and through various successive processes, and semiconductor chips are formed on the wafer. In an example of such a photomask, chromium is coated on a transparent mask plate of glass for forming an opaque pattern layer. Such a photomask is called a hard mask. In a master mask, or a copying mask for actual use composed of the hard mask, a defective portion, such as a portion having an excessive opaque coating or a portion lacking a part of a shape, should be amended. In a conventional method of amendment, first, a photomask to be amended is checked by an automatic inspection device, so as to obtain datum of the position of the defective portion. Then, this photomask is set in an amending apparatus, such as a spot exposure device, and the datum of the position of the defective portion is input into the apparatus so that the defective portion can be amended as follows. The photomask is pre-coated with a photo-resist agent before the mask is set in the spot exposure device. The spot exposure device contains a microscope. The defective portion of the mask is automatically positioned in the view field of the microscope is accordance with the position datum. The defective portion is viewed by an operator for confirming the shape and the position of the portion. The device comprises an adjustable rectangular aperture through which a light passes so as to irradiate a visible spotlight which does not sensitize the photo-resist on the mask. The size and the position of the rectangular aperture are precisely adjusted so as to correspond to that of the defective portion. Then, an exposure spotlight is irradiated onto the defective portion through the adjusted aperture. After that, the resist agent is developed so as to add or remove the opaque coating at the defective portion.
However, in the above method, the aperture for irradiating the spotlight is positioned by aligning an edge of the rectangular shape of the spotlight with a linear part of the profile of the defective shape of the pattern component. Therefore, it is impossible to amend a defective pattern component if the profile is not clearly defined or if the component is completely omitted, because there is no base line with which an edge of the spotlight can be aligned.